Improvement in satellite broadcasting service has recently be demanded. Accordingly, the number of channels used for satellite broadcasting has increased. A single LNB (Low Noise Block Downconverter) receives radio waves transmitted from respective ones of a plurality of satellites. In addition, a circuit such as an LNB including a plurality of output terminals for tuning serving for connection to a tuner and an SW (Switch)-BOX including a plurality of input ports and output ports for IF (Intermediate Frequency) has a complicated structure.
In an SW-BOX in which a microstrip line is shielded by a chassis 300 and a frame 200 as shown in FIG. 7, a larger number of signal lines should be provided on a dielectric substrate 01, than in a conventional example.
When a large number of signal lines 102a, 102b, and 102c are provided on one main surface of dielectric substrate 101 as shown in FIG. 8, a problem of electro-magnetic interference between signal lines 102a and 102b as well as electro-magnetic interference between signal lines 102c and 102b arises. In order to suppress the electro-magnetic interference among signal lines and in order to prevent electro-magnetic interference of an external electro-magnetic wave to the signal line, conductive layers attaining a function as ground such as ground patterns 103a, 103b, 103c, and 103d in FIG. 8 are provided. Ground patterns 103a and 103d are provided in order to prevent the electro-magnetic interference of the external electro-magnetic wave to the signal line. Ground patterns 103a, 103b, 103c, and 103d are connected to a ground pattern 50 through a plurality of plugs (such as through holes) 104 penetrating dielectric substrate 101, so that potentials of the ground patterns are fixed to a prescribed level.
In the microstrip line as described above, further increase in the number of signal lines has been desired. To that end, the ground pattern should have a further smaller width. On the other hand, if the width of the ground pattern is too small, the ground pattern will no longer be able to suppress the electromagnetic interference among the signal lines, and accordingly, the width of the ground pattern cannot be made smaller. Instead, an area of the main surface of the dielectric substrate should be increased. Increase in a size of the dielectric substrate, however, is contrary to the demand for a smaller size of the microstrip line.